linux-kernel - Re: [PATCH v8 24/33] x86/fred: disallow the swapgs instruction when FRED is enabled

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Date:   Mon, 05 Jun 2023 15:47:20 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, peterz@...radead.org, andrew.cooper3@...rix.com,
        seanjc@...gle.com, pbonzini@...hat.com, ravi.v.shankar@...el.com,
        jiangshanlai@...il.com, shan.kang@...el.com
Subject: Re: [PATCH v8 24/33] x86/fred: disallow the swapgs instruction when
 FRED is enabled

On Mon, Apr 10 2023 at 01:14, Xin Li wrote:
> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
>
> The FRED architecture establishes the full supervisor/user through:
> 1) FRED event delivery swaps the value of the GS base address and
>    that of the IA32_KERNEL_GS_BASE MSR.
> 2) ERETU swaps the value of the GS base address and that of the
>    IA32_KERNEL_GS_BASE MSR.
> Thus, the swapgs instruction is disallowed when FRED is enabled,
> otherwise it causes #UD.

Which does not explain why writing directly to the IA32_KERNEL_GS_BASE
MSR is doing the right thing.

Thanks,

        tglx